An electrolytic copper foil is generally produced as follows. A rotating metal cathode drum with a polished surface is used along with an insoluble metal anode that surrounds said cathode drum and is disposed at a position substantially corresponding to the lower half of said cathode drum, a copper electrolytic solution is allowed to flow between the cathode drum and the anode, a potential differential is provided between these to electrodeposit copper onto the cathode drum, and the electrodeposited copper is peeled away from the cathode drum at the point of reaching a specific thickness, so that a copper foil is produced continuously.
A copper foil obtained in this way is generally called a raw foil, and after this it is subjected to a number of surface treatments and used for printed wiring boards and so forth.
FIG. 1 is a simplified diagram of a conventional apparatus for producing a copper foil. This electrolytic copper foil production apparatus has a cathode drum 1 installed in an electrolysis bath containing an electrolytic solution. This cathode drum 1 is designed to rotate while being partially submerged (substantially the lower half) in the electrolytic solution.
An insoluble anode 2 is provided so as to surround the outer peripheral lower half of this cathode drum 1. A specific gap 3 is maintained between the cathode drum 1 and the anode 2, and an electrolytic solution is allowed to flow through this gap. Two anode plates are disposed in the apparatus shown in FIG. 1.
With the apparatus in FIG. 1, the electrolytic solution is supplied from below, and this electrolytic solution goes through the gap 3 between the cathode drum 1 and the anode 2, overflows from the top edge of the anode 2, and is then recirculated. A rectifier is interposed between the cathode drum 1 and the anode 2 so that a specific voltage can be maintained between the two components.
As the cathode drum 1 rotates, the thickness of the copper electrodeposited from the electrolytic solution increases. When at least a certain thickness is reached, this raw foil 4 is peeled away and continuously taken up. A raw foil produced in this manner is adjusted for thickness by varying the distance between the cathode drum 1 and the anode 2, the flow rate of the supplied electrolytic solution, or the amount of electricity supplied.
A copper foil produced with an electrolytic copper foil producing apparatus such as this has a mirror surface on the side touching the cathode drum, but the opposite side is a rough surface with bumps and pits. Problems encountered with ordinary electrolysis are that the bumps and pits on the rough side are severe, undercutting tends to occur during etching, and fine patterning is difficult.
On the one hand, as the density on printed wiring boards has steadily risen, there has more recently been a need for a copper foil that can be more finely patterned as circuit width decreases and multilayer circuits are produced. This fine patterning requires a copper foil that has a good etching rate and uniform solubility, that is, a copper foil with excellent etching characteristics.
On the other hand, the performance needed in a copper foil used for printed wiring boards is not just its elongation at ordinary temperature, but also its high-temperature characteristics for preventing cracking caused by thermal stress, as well as high tensile strength for good dimensional stability in a printed wiring board. However, copper foil in which the bumps and pits of the rough surface side are severe as mentioned above has the problem of being totally unsuited to fine patterning, as discussed above. Because of this, smoothing the rough side to a low profile has been investigated.
It is known that achieving a low profile generally can be accomplished by adding a large amount of glue or thiourea to the electrolytic solution.
Nevertheless, a problem with such additives is that they sharply decrease the elongation at ordinary and high temperatures, which greatly lowers the performance of the copper foil when used for a printed wiring board.
It has also been proposed that the elongation characteristics of the resultant copper foil can be improved by using an adduct salt of a polyepichlorohydrin and a tertiary amine as an additive to a copper plating solution (Specification of U.S. Pat. No. 6,183,622).
However, the inventors have confirmed that this method actually results in deterioration of elongation characteristics, and does not contribute to achieving a lower profile.